Outdated install guide

[webhive]

If literally follow your instructions on main page, getting this

So version on rubygems is old like mammouth shit.

I don't know what is a way or resolve it better matched to you. Options could be

Update home page readme to use

gem "solidus_virtual_gift_card", github: "solidusio-contrib/solidus_virtual_gift_card"

instead of just

gem "solidus_virtual_gift_card"

Or updated rubygems version to actual one

[kennyadsl]

@webhive you are welcome to make a PR to update the readme, thanks!